The present invention relates to the examination of an object, such as a human body, for example, and possibly to the simultaneous guidance of a treatment procedure.
The nuclear magnetic resonance phenomenon (NMR) has already been applied in medicine to magnetic resonance imaging (MRI) for more than ten years. The design of various pieces of magnetic imaging equipment and the use of various techniques have been dealt with in a number of books and the latest research results are currently published in several scientific journals focusing exclusively on that particular field.
A common feature for all magnetic imaging devices is the positioning of an object to be imaged, often a patient, in a stationary magnetic field B0↑ which is produced by a magnet. In addition to this, the magnetic field is subjected to a linear magnetic-field change, a gradient, which is effected by means of a special gradient coil. The magnetic imaging devices are provided with three gradients Gx, Gy, and Gz, which represent the change of a magnetic field in the direction of an x, y, and z axis, respectively. The gradients are used for encoding positional information from the magnetically resonating material, most commonly protons, of an object to be imaged, by frequency-modulating the resonance. The signal for magnetic imaging is produced by means of radio-frequency (RF) coils, which excite the resonance and function as a signal receiver. The signal is analyzed (Fourier-transformed) for its frequency content, thereby determining a signal distribution in the direction to be examined. The literature discloses a variety of methods for applying this basic technique for producing 2- or 3-dimensional images by using special imaging sequences, which are all based on the encoding of an NMR signal effected by means of gradients in x, y, and z directions.
The open-configuration magnetic imaging equipment offers a possibility of performing procedures, for example biopsies, during patient imaging. Certain objects, for example brain tumors, osteopecilia, soft tissue transformations, or liver tumors, are best discernible in magnetic imaging and, thus, the procedure would be most precisely applicable in the guidance of magnetic imaging. However, this requires that the position of an operation instrument in a tissue during the procedure be known with high accuracy.
Prior known are operation instruments disclosed in reference Longmore: U.S. Pat. No. 4,827,931, which are completely or partially made of a material visible in a normal magnetic image. Prior known are also methods disclosed in references Werne R: WO 98/22022, Werne R: U.S. Pat. No. 5,782,764, Ratner A: U.S. Pat. No. 4,989,608, Ratner A: U.S. Pat. No. 5,154,179, wherein an operation instrument is provided either in a container or otherwise with an NMR active substance, having a relaxation time which is different from that of the tissue, for creating a contrast distinction between the operation instrument and the tissue. It is characteristic of these methods that the NMR signal emitted by these instruments is not enhanced, but the visibility is only based either on a high density (e.g. water) of the nuclei emitting the NMR signal or on a relaxation time other than that of the tissue. Thus, the visibility of these instruments in a tissue is weak and requires that the imaging process be performed by using either very thin slices or by selecting such imaging sequences that the surrounding tissue provides very little signal. The use of thick slices in imaging is desirable, e.g. when monitoring the progress of an instrument in a curved blood vessel or generally when using pliable instruments, e.g. thin needles. In terms of monitoring a procedure, it is preferable to obtain sufficient signal from a tissue in order to make the surrounding tissue visible simultaneously with the instrument. There are prior known extra-object stereotactical frames as set forth in reference Cosman E: U.S. Pat. No. 4,618,978, which enable determining the position of the object in a tissue. Prior known is the use of operation instruments disclosed in references: Mueller P et al.: MR-guided aspiration biopsy: needle design and clinical trials, Radiology 161 pp. 605-609 (1986), Lufkin R et al.: MR body stereotaxis: an aid for MR-guided biopsies, Journal of Computer Assisted Tomography 6 pp. 1088-1089 (1988), van Sonnenberg E, et al.: A wiresheath system for MR-guided biopsy and drainage, AJR 151 pp. 815-817 (1988), Lufkin R. Teresi L, Chiu L, Hanafee W: A technique for MR-guided needle placement, AJR 151 pp. 193-196 (1988), Bakker C, Hoogeveen R, Weber J, et al.: Visualization of dedicated catheters using fast scanning techniques with potential for MR-guided vascular interventions, Cordington R: U.S. Pat. No. 4,572,198, Magnetic Resonance in Medicine 36 pp. 816-820 (1996), Glowinski A, Adam G, Bucker A, et al.:Catheter visualization using locally induced, actively controlled field inhomogeneities, Magnetic Resonance in Medicine 38 pp. 253-258 (1977), which cause the weakening of an image signal from the area of an operation instrument and its immediate vicinity. Also prior known is an instrument set forth in reference Werne R: U.S. Pat. No. 5,744,958, which is fitted with a conductive foil. The instrument is not provided with its own source for an NMR-frequency signal, nor is the foil transparent to an NMR-frequency signal, but the instrument has its visibility based on the distortion of a surrounding-tissue emitted signal in the vicinity of the instrument, and the intensity of distortion is controlled by the foil thickness. The positional information produced by the above-described operation instruments is generally perceived as a loss of the signal, while positive contrast would be desirable.
According to reference Dumoulin C: U.S. Pat. No. 5,419,325, it is prior known to fit the instrument with a Faraday shield for protection against external RF excitation. Prior known is a method disclosed in reference Young I: U.S. Pat. No. 5,409,003, wherein the nuclei emitting an image signal perform limited motion in the vicinity of the surface of an instrument, and no fading of the signal as a result of diffusion shall occur. Reference Yates D: U.S. Pat. No. 5,188,111 anticipates an instrument, having a stem portion which is pliable in a tissue and whose end carries a sensor capable of tracking action.
Prior known is a method described in references Dumoulin et al.: U.S. Pat. No. 5,271,400, Dumoulin et al.: U.S. Pat. No. 5,307,808, Dumoulin et al.: U.S. Pat. No. 5,318,025, Souza et al.: U.S. Pat. No. 5,353,795, Leung D A, et al.: Intravascular. MR tracking catheter, AJR 164 pp. 1265-1270 (1995), wherein the operation instrument is fitted with one or more small-sized RF coils, whose position is detected by means of an imaging method applied in magnetic imaging. Furthermore, references Darrow et al.: U.S. Pat. No. 5,445,151, Dumoulin et al.: U.S. Pat. No. 5,447,156 describe, as an application of this technique, the imaging of blood vessels and the measurement of parameters associated with circulation. Reference Young I: U.S. Pat. No. 5,303,707 discloses a method, wherein a small-sized gradient coil is attached to the instrument. Prior known is also a method according to reference Ocali O. et al: Intravascular magnetic resonance imaging using a loopless catheter antenna, Magnetic Resonance in Medicine 37 pp. 112-118 (1997), wherein the catheter functions as NMR-frequency antenna and gathers a tissue signal from the immediate vicinity of the antenna. Prior known are methods set forth in references: Silverman S et al.: Interactive MR-guided biopsy in an open-configuration MR-imaging system, Radiology 197 pp. 175-181 (1995), as well as Roemer P. et al: U.S. Pat. No. 5,365,927, wherein fixing the position of an operation instrument is based on detecting the position of small transmitters mounted on the stem portion. One prior known method is described in reference Kaufman L: U.S. Pat. No. 5,155,435, wherein the position of an operation instrument is projected on top of a previously acquired anatomical image. The above methods are sensitive to motion, as well as to the inhomogeneities of a magnetic field and gradient fields, since the imaging of the anatomical structure of an object is carried out by a method other than the one used for determining the position of an operation instrument. Prior known is a method disclosed in reference Golman K, Leunbach I, Adrenkjaer-Larsen et al.: Overhauser-enhanced MR imaging (OMRI) Acta Radiologica 39 pp. 10-17 (1997), wherein the blood circulation or a tissue is injected with a contrast medium or marker, the signal emitted thereby being enhanced by making use of dynamic nuclear polarization. Reference Sepponen R: U.S. Pat. No. 5,211,166 describes a principle, wherein the NMR-frequency signal emitted by a marker sample placed in an operation instrument is enhanced by means of dynamic nuclear polarization. Prior known is a stereotactical frame, disclosed in reference Sepponen R: U.S. Pat. No. 5,218,964 and based on the same principle. Prior known methods are set forth in references Joensuu R, Sepponen R. Lamminen A, Standertskjxc3x6ld-Nordenstam C-G, Interventional MR imaging: Demonstration of the feasibility of the Ovcerhauser marker enhancement (OMEN) technique, Acta Radiologica 38 pp. 43-46 (1997), Joensuu R, Sepponen R, Lamminen A, Savolainen S, Standertskjxc3x6ld-Nordenstam C-G, Hieh-accuracy MR tracking of interventional devices: the Overhauser marker enhancement (OMEN) technique, Magnetic Resonance in Medicine 40(6) pp. 914-921 (1998), wherein dynamic nuclear polarization is used for enhancing an NMR-frequency signal emitted by a marker sample, which is placed in an operation instrument and which is electromagnetically open with respect to its environment.
Dynamic nuclear polarization (DNP) is a magnetic double-resonance method, which thus necessitates two separate spin populations. Such spin populations are constituted for example by the spins of electrons and protons. In DNP, a paramagnetic material, hereinafter an enhancer, functions as a source for the transitions of an electron spin resonance (ESR) and it is interaction, generally in a blended form, with an NMR-signal emitting medium, for example water. This blend or composition, hereinafter a contrast medium or a marker, is exposed to a first radiation, having its frequency and amplitude selected in view of exciting some or all of the ESR transitions of the enhancer (this is generally a microwave-range frequency and, thus, such radiation is hereinafter referred to as microwave radiation). In order to produce a magnetic image, the object is exposed to a second radiation, having its frequency selected in view of exciting the nuclear spin transitions in imaging nuclei. The result is an improved magnetic image of a marker-containing object, the image having its signal strength enhanced with a factor that may be 100 or higher.
Dynamic nuclear polarization and enhancers applicable therein are dealt with e.g. in references Potenza, J.: Measurement and applications of dynamic nuclear polarization. In: Advances in molecular relaxation processes, 4 pp. 229-354, Elsevier Publishing Company, Amsterdam (1972), Leunbach I: U.S. Pat. No. 5,314,681, Andersson S. et al.: U.S. Pat. No. 5,530,140, Golman K: U.S. Pat. No. 5,435,991.
References Leunbach I: U.S. Pat. No. 4,984,573 and Leunbach I: U.S. Pat. No. 5,203,332 disclose DNP-compatible magnetic imaging devices, and reference Ehnholm G: U.S. Pat. No. 5,184,076 discloses a DNP-compatible imaging coil configuration. Reference Maciel G, Davis M: NMR imaging of paramagnetic centers in solid via dynamic nuclear polarization, Journal of Magnetic Resonance 64 pp. 356-360 (1985) describes a method fit for the mapping of paramagnetic components by combining the DNP and MRI methods. Reference Lurie et al.: U.S. Pat. No. 4,891,593 shows the mapping of paramagnetic components as an application. Reference Ettinger K: U.S. Pat. No. 4,719,425 discloses, as applications, the mappings of the contents of paramagnetic components and the activity of cerebral nerve cells. Reference Lurie D, Bussel D, Bell L, Mallard J: Proton-electron double magnetic resonance imaging of free radical solutions, Journal of Magnetic Resonance 76 pp. 366-370 (1988) discloses, as possible applications, the mappings of free radical groups, nitroxide radicals, and degree of oxidation.
A fundamental benefit gained by DNP the NMR-signal-to-noise ratio of an object can be greatly improved. By virtue of this, the marker-containing object can be very small in size, and yet it has a better visibility in a magnetic image than for example the surrounding tissue.
However, DNP has its limitations. Since the resonance line of ESR has a considerable width, the saturation of a marker requires the use of a high power, which in current technology can only be produced by using expensive power amplifiers. Another problem is the tendency of microwave radiation to heat up the object. Hence, with an object of a given size and shape and with a given imaging capacity, the power Pa absorbed by the object is proportional to the squares of the amplitude Am and frequency fm of microwave radiation. Consequently,
xe2x80x83Pa=kofm2Am2Dmxe2x80x83xe2x80x83(1)
wherein ko is a constant and Dm is the active period of microwave radiation, which, as a result of being absorbed in a tissue, may cause damage thereto (excessive heating).
The interaction between electromagnetic radiation and a biological tissue has been dealt with e.g. in reference: Rxc3x6schmann P: Radiofrequency penetration and absorption in the human body: Limitations to high field whole body nuclear magnetic resonance imaging, Medical Physics 14(6) pp. 922-931 (1987).
In practice, the tracking of a catheter, a surgical instrument, a biopsy needle, or a radiotherapeutical charge, generally an operation instrument, with respect to various tissues must be performed at a precision as high as possible. Hereinbelow, the location of an instrument interesting at any given moment will be referred to as an operation site. This requires that a high contrast and a good signal-to-noise ratio be achieved in imaging between said instrument or a part thereof and the tissue of an operation site. In addition, the operation instrument must not damage the tissue more than what is necessary for guiding the instrument to the operation site. Furthermore, the operation instrument must be reliable in operation in all conditions, readily cleanable, and easy to use, as well as preferably attractive in terms of its price.
The objectives of the invention are achieved on the basis of claim 1 or claim 2. Preferred embodiments are set forth in the subclaims.